First, is that all pre-existing temporary networks should get new
unique identifiers. So XC from 1992 should get a network code that
identifies it as distinct from XC in 1993. Obviously some backwards
compatibility is important, so the code XC should never again be used
but XC in a data requests for a time window in 1992 should still work,
but would be nice to have a path so that eventually the old two
character temporary network codes are deprecated. So the proposal
should include a mapping from existing temporary codes into the new 6
digit codes. A natural mapping would be something like XC from 1992
gets XC92 or XC1992 or 1992XC.

Second, if there is a mapping and it follows the above proposal,
should we worry about Y2K type issues go ahead and use the 4 digit
year? I believe there is plenty of room in the 6 char space to keep
the full year as the 2 remaining characters give up to 36x36=1296
temporary network codes per year, which seems plenty. I did not do a
complete search, but there is at least an XA from 1969 (lunar
seismometers), 53 years is a while, but might as well future proof
this as much as reasonable.

Third, there is an advantage of reversing the mapping so that the year
comes first and the characters come after. In this way, we follow the
existing "XYZ or digit as first character means temporary network"
convention, but the first character being a year is always a number.
This also makes the mapping unique and easy for old temporary
networks, prepend the start year. And because the start of they year
will always be a 1 or a 2, frees up all network codes that start with
0,3,..,9 and are longer than 2 characters to be held in reserve for
future use. Maybe all permanent martian networks will start with the
number 4. Some of this may be beyond what we need to worry about, but
6 chars is big enough I would recommend at least holding a part of the
network code space back for future unknown uses.